This section provides background information related to the present disclosure, which is not necessarily prior art.
Recently, a 3 dimensions multiple input multiple output (3D MIMO) system is increasingly popular, since a vertical dimension is considered in the 3D MIMO system. The 3D MIMO system brings the following two advantages: firstly, saving an arrangement space of a large-scale antenna; and secondly, providing another frequency selective dimension for inter-cell interference coordination (ICIC).
The 3D MIMO system is mainly applied to a high building scene and an indoor scene. For the high building scene, a macro cell arranged outside the building transmits to a user in the building using a 3D wave beam. The indoor scene is also an object for application.
In the 3D MIMO system, a position of a user is described in a 3D mode, which describes the position of the user more accurately than the previous 2 dimensions (2D) MIMO system. However, in the existing 3D MIMO technology, it is inevitable to interfere with a neighbor cell. Hence, it needs to sufficiently utilize the advantage of the 3D MIMO technology and perform inter-cell interference coordination.
In addition, high overhead of the reference signal severely limits the performance of the 3D MIMO system. A study report from the China Mobile points out that, overhead of the reference signals of a 8 antenna port, 64 antenna port and 128 antenna port accounts for 1%, 8% and 16% of the total overhead respectively.
The 3Gpp Rel-8 standard only defines the maximum 4 antenna port, while the Rel-10 extends the maximum antenna port to 8. Accordingly, in order to reduce overhead of the reference signal, the Rel-10 decompose a part of functions of a common reference signal (also referred to as cell-specific reference signal, CRS) into channel state information reference signal (CSI-RS) and demodulation reference signal (DMRS), which correspond to channel estimation and demodulation respectively.
For the long term evolution (LTE), it is stipulated in the 3D MIMO system that at least 32 antennae need to be used. Therefore, it is attracting to improve the system performance utilizing the existing reference signal (RS) structure (the RS structure before Rel-12) in the 3D MIMO system.